A gas ratio control device of the above-described class is shown in U.S. Pat. No. 4,972,831 (corresponding to DE-A1-38,10,745).
In the prior-art gas ratio control device, an anesthetic gas is carried in an anesthetic gas feed line which contains a pressure reducer and a control valve, which precede an anesthetic gas adjusting valve. The metered anesthetic gas flows via a first measuring resistance and a flow-measuring tube for indicating the metering to a fresh gas outlet. The oxygen is carried via an oxygen feed line to this fresh gas outlet, which contains a pressure reducer and an oxygen adjusting valve, as well as a second measuring resistance and a flow-measuring tube. The gas streams generate dynamic pressures at the measuring resistances; these dynamic pressures are sent to a proportional member which controls the control valve in the anesthetic gas feed line depending on the value of the difference between the two dynamic pressures so that the anesthetic gas flow cannot exceed a certain value in relation to the oxygen flow. It is thus ensured for the patient that the fresh gas fed in will contain at least a defined ratio of oxygen concentration, e.g., 25 vol. %. The detailed design of such a proportional valve is described in EP-B1-39932.
Before the fresh gas enters the respiration system, it is also passed through an anesthetic vaporizer which enriches the fresh gas with anesthetic vapor. The additional volume percent of anesthetic in the fresh gas leads to a certain reduction of the oxygen concentration compared with the value determined in front of the gas ratio control device. If a low volume percent of anesthetic is metered in, the deviation is slight and is normally on the order of magnitude of the tolerance range of the gas ratio control device. This will be illustrated on the basis of a numerical example. If a halothane stream of 0.4 L is mixed with a fresh gas stream consisting of 2.5 L oxygen and 7.5 L laughing gas, the percentage of oxygen will decrease from 25% without halothane to 24% with halothane. A halothane stream of 0.4 L means in this case a setting of 4 vol. % on the metering adjusting member of the anesthetic vaporizer, which corresponds to the usual maximum concentrations for halothane. If, in contrast, anesthetics are used which require higher concentrations in the fresh gas in order to obtain the desired medical effect, the reduction of the oxygen concentration cannot be ignored any longer. If 2.5 L of another anesthetic is metered in instead of 0.4 L halothane, the oxygen concentration will drop from 25% to 20%. The possible solution that presents itself, namely, changing the basic setting of the gas ratio control device to a correspondingly high minimum oxygen concentration value, is ruled out, because the analgesic effect of the anesthetic gases is not fully utilized in the case of anesthetics to be added in low doses.